1. Field of the Invention
This invention relates generally to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to an improved antiparallel-pinned spin valve sensor, and to magnetic recording systems which incorporate such sensors.
2. Description of Related Art
Computer systems generally utilize auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material separated by a layer of non-magnetic electrically conductive material are generally referred to as spin valve (SV) sensors manifesting the GMR effect (SV effect). In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In SV sensors, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. It should be noted that the AMR effect is also present in the SV sensor free layer and it tends to reduce the overall GMR effect.
FIG. 1 shows a typical SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. The central region 102 has defined edges and the end regions are contiguous with and abut the edges of the central region. A free layer (free ferromagnetic layer) 110 is separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer 115. The magnetization of the pinned layer 120 is fixed through exchange coupling with an antiferromagnetic (AFM) layer 121. Free layer 110, spacer 115, pinned layer 120 and the AFM layer 121 are all formed in the central region 102. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed over hard bias layers 130 and 135, respectively, provide electrical connections for the flow of the sensing current I.sub.s from a current source 160 to the MR sensor 100. Sensing means 170 connected to leads 140 and 145 sense the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g., field generated by a data bit stored on a disk). IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al. and incorporated herein by reference, discloses an MR sensor operating on the basis of the SV effect.
Another type of spin valve sensor currently under development is an antiparallel (AP)-pinned spin valve sensor. FIG. 2A show an AP-Pinned SV sensor 200 which was the subject of several experiments conducted by the inventor. SV sensor 200 has end regions 202 and 204 separated from each other by a central region 206. AP-pinned SV sensor 200 comprises a Ni-Fe free layer 225 separated from a laminated AP-pinned layer 210 by a copper spacer layer 220. The magnetization of the laminated AP-pinned layer 210 is fixed by an AFM layer 208 which is made of NiO. The laminated AP-pinned layer 210 includes a first ferromagnetic layer 212 (PF1) of cobalt and a second ferromagnetic layer 216 (PF2) of cobalt separated from each other by a ruthenium (Ru) antiparallel coupling layer 214. The AMF layer 208, AP-pinned layer 210, copper spacer 220, free layer 225 and a cap layer 230 are all formed sequentially in the central region 206. Hard bias layers 235 and 240, formed in end regions 202 and 204, provides longitudinal biasing for the free layer 225. Electrical leads 245 and 250 are also formed in end regions 202 and 204, respectively, to provide electrical current from a current source (not shown) to the SV sensor 200. As shown in FIG. 2B, the magnetization direction 265 (MF) of the free layer 225 is set to be parallel to the air bearing surface (ABS) in the absence of an external field. The magnetizations directions 255 (M.sub.P1) and 260 (M.sub.P2)of the pinned layers 212 and 214, respectively, are also set to be perpendicular to the ABS.
A key advantage of the AP-pinned SV sensor of FIG. 2 is the improvement of the exchange coupling field strength between the AFM layer 208 and AP-pinned layer 210. This improved exchange coupling increases the stability of the AP-pinned SV sensor 200 at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer 208.
However, there are two major issues associated with the AP-pinned SV sensor of FIG. 2A. First, experiments conducted on the SV sensor 200 have shown that the read signal asymmetry of the AP-pinned SV sensor 200 changes from near 0% to about 30% as the thickness of the free layer 225 was changed from 72 .ANG. to 45 .ANG. while the thickness of the other layers were left unchanged. Read signal asymmetry is defined by the relation ##EQU1## where V.sub.1 is the maximum positive voltage of the read signal and V.sub.2 is the maximum negative voltage of the read signal and max(V.sub.1,V.sub.2) means the maximum of either V.sub.1 or V.sub.2, whichever is larger. FIGS. 2C and 2D show the transfer curves for the read back voltage signal as a function of the applied fields for the SV sensor 200 with free layer thickness of 72 .ANG. and 45 .ANG., respectively. Such a large read signal asymmetry implies that the optimum quiescent-state bias point does not lie near the middle of the resistance versus external field curve (FIG. 2D). The non-optimum bias point can cause sensor saturation, which results in non-linear response by the SV sensor. In-addition, the sensor saturation can also cause magnetic instability in the sensor. Due to these unfavorable effects, the error rate performance of the sensing means (for example, the recording channel) could be unacceptably degraded.
The reason for such a large signal asymmetry in the shape of the read signal sensed by the SV sensor 200 having a free layer thickness of 45 .ANG. is due to the fact that the bias point of the AP-pinned SV sensor is determined by the balance of several forces (H.sub.DemagAp, H.sub.FC, H.sub.I, and AMR) acting on the free layer 225 as shown in FIG. 2B. The demagnetization field (H.sub.DemagAP) of the pinned layer 210 and the ferromagnetic coupling field (H.sub.FC) between the free and pinned layers 225 and 210 is near zero. As a result, the bias point is mainly determined by the balance between the AMR effect present in the free layer 225 and the field generated by the current (H.sub.I) flowing in the pinned layer 210 and the spacer layer 220. For a 72 .ANG. thick free layer 225, the AMR coefficient is about 1% and the effect of the AMR on the bias point is nearly balanced by the H.sub.I field from the sense current flowing in the pinned layer and the spacer layer. However, for a 45 .ANG. thick free layer 225, the AMR coefficient is only 0.5%. As a result, the effect of the AMR on the bias point is not balanced by the field H.sub.I from the sense current.
Second, the exchange coupling field between the AFM layer 208 and the AP-pinned layer 210 is inversely proportional to the magnetic moment difference (net magnetic moment) between the two AP-pinned ferromagnetic layers 212 and 216. However, it is very difficult to control the net moment of the AP-pinned layer 210 (Co/Ru/Co) because of interfacial diffusion that takes place at the interface between the NiO AFM layer 208 and the first pinned layer 212 of Co. This interaction between the NiO AFM layer 208 and the Co first pinned layer 212 creates magnetic dead layer at the NiO/Co interface. The interfacial diffusion that takes place at the aforementioned interface causes a change in the moment of the first pinned Co layer 212 even after the AP-pinned SV sensor of FIG. 2A has been completely built. The change in the moment of the first pinned layer 212 causes the change in the net moment of the AP-pinned layer 210 by factors of 2 to 3 from one wafer to another. Such large variations in the net moment of the AP-pinned layer 210 results in large variations in the pinning fields which compromises the stability of the SV sensor 200 as well as the symmetry and size of the signals detected (read) by the sensor.
Aside from the aforementioned major issues associated with the AP-pinned SV sensor 200, disk drive industry has been engaged in an ongoing effort to increase the GMR coefficient of the SV sensors in order to store more and more bits of information on any given disk surface.
Therefore, there is a need for an AP-pinned SV sensor where the symmetry of the read signal detected by the SV sensor (read signal symmetry) is substantially improved as well as having a large GMR coefficient.